Amino acid and putative neurotransmitter transport in human Y79 retinoblastoma cells. Effect of insulin and insulin-like growth factor

Identification and functional characterization of riboflavin transporter in human-derived retinoblastoma cell line (Y-79): mechanisms of cellular uptake and translocation

Drug delivery to the retina is a challenging task owing to its complex physiology and presence of the blood-retinal barrier (BRB), which regulates the permeation of substances from blood into the retina. Transporter-targeted drug delivery has become a clinically significant drug-delivery approach for enhancing the bioavailability of various drugs. Different nutrient transporters have been reported to be expressed on the retina. Riboflavin (vitamin B2), an essential nutritional vitamin for the development and maintenance of the surface structures and functions of epithelial cells of the ocular tissues, must be acquired from retinal or choroidal blood supply. The uptake mechanism, cellular translocation, and major regulatory pathways of riboflavin uptake into retina are poorly understood. Therefore, the aim of this study was to investigate the presence of a riboflavin transporter and delineate uptake and intracellular trafficking of riboflavin in the human-derived retinoblastoma cell line (Y-79), a model for neural retina. Uptake characteristics of [3H]riboflavin in Y-79 cells were found to be (1) linear with time over 10 min of incubation; (2) temperature- and energy-dependent; (3) sodium, chloride-, and pH-independent; (4) concentration dependence with an apparent K(m) of 19.21 +/- 0.37 nM and V(max) of 6.98 +/- 0.30 pmol/min/mg protein; (5) inhibited by the structural analogs (lumiflavin and lumichrome) but not by the structurally unrelated vitamins; and (6) uptake of [3H]riboflavin was trans-stimulated by the intracellular riboflavin. Neither protein kinase C- nor protein tyrosine kinase-mediated pathways were involved in regulating riboflavin uptake. However, protein kinase A pathway activators (IBMX and forskolin) and inhibitors (H-89) and Ca2+/calmodulin pathways appeared to play important roles in the regulation of riboflavin uptake in Y-79 cells through significant reduction in V(max) (39%) and significant increase in K(m) (112%) of the uptake process. These studies demonstrated, for the first time, the existence of a specialized carrier-mediated system for riboflavin uptake in human-derived retinoblastoma cells. The system appears to be regulated by protein kinase A and Ca2+/calmodulin pathways. Being a high-affinity low-capacity transport system, the presence of this transporter on the retina may be suitable for the design of transporter-targeted prodrugs to achieve enhanced permeability for highly potent, but poorly bioavailable, compounds where a small increase in the bioavailability could result in a significant increase in therapeutic response.

Role of insulin-like growth factors in autocrine growth of human retinoblastoma Y79 cells

In this study, we have demonstrated that human retinoblastoma Y79 cells produce insulin-like growth factors (IGFs) type I and type II and release them into the medium. We have also ascertained, by means of competitive studies and cross-linking procedure, that Y79 cells contain the type-I IGF receptor (IGF-IR). Furthermore, surface-bound IGF-I is internalised by the receptor, then degraded to amino acids. Insulin, IGF-I and IGF-II caused down-regulation of IGF-IR; the effect is concentration and time dependent. Scatchard analysis demonstrated that incubation with insulin markedly decreased the binding capacity measured for IGF-I while the apparent Kd value calculated for IGF-I binding was not significantly modified. IGF-I, IGF-II and insulin induced tyrosine phosphorylation of IGF-IR. Tyrosine phosphorylation of this receptor with, however, a less strong signal, was detectable even in cells cultured in serum-free medium without the addition of any exogenous growth factor. Similar results have been found concerning the tyrosine phosphorylation of insulin receptor substrate-1 (IRS 1). Tyrosine phosphorylation of both IGF-IR and IRS 1, either under basal conditions or after stimulation with growth factors, was strongly inhibited when alpha-IR3, a monoclonal antibody to IGF-IR, was added to the culture. IGF-I was capable of inducing Y79 cell proliferation and its effect was entirely inhibited by the addition of alpha-IR3. This antibody also markedly reduced the proliferation of Y79 cells cultured in serum-free medium not supplemented with stimulatory factors. Our results indicate that IGF-I and IGF-IR mediate an autocrine growth mechanism in Y79 cells.

Retinoblastoma cells in tissue culture

This review summarizes and discusses research on retinoblastoma (Rb) cells in tissue culture. Retinoblastoma is an intraocular tumor of early childhood which is believed to originate from the primitive multipotential neuroectoderm of the optic cup region. The application of tissue culture techniques to the study of Rb cells permits detailed studies of the biology of this tumor. Classic studies have primarily focussed on growth and metastatic potential of Rb cells. Y-79 Rb cells, for example, have a short doubling time in vitro as well as aggressively growing in the anterior chambers of athymic 'nude' mice. Such active growth may result from secretion of a Retinoblastoma Derived Growth Factor (RDGF) by the cells. Several natural agents have now been shown to halt Rb cell growth in vitro. Among these are the fatty acid, butyrate, and two retinoids: retinol and retinoic acid. Interestingly, the retinoids have different mechanisms of action. Cultured Y-79 and WERI cells appear to be multipotential in that they exhibit both neuronal- and glial-like characteristics. Natural agents such as cyclic AMP and butyrate can induce the cells to differentiate along either neuronal or glial cell lines as assessed morphologically and immunocytochemically. Of interest is that combination of agents such as butyrate and laminin, an extracellular attachment protein, yield totally different morphologies, in this case, pigment epithelial in nature. Tissue culture studies thus not only show the primitive, multipotential nature of the Rb cells but their great plasticity as well. Such studies are also useful in elucidating the multiple factors (e.g., substrata and soluble agents) which code for normal retinal development from embryo to adult.